1. Field of the Invention
The present invention relates to a process for drawing and stretching sheet metal and particularly to such a method for forming a stretch drawn work hardened boat hull from corrosion resistant aluminum.
2. The Prior Art
Processes for forming shapes from sheet metal by stretch drawing are well known. These processes generally involve forming a preform by placing a work piece consisting of a piece of sheet metal between the bulldozer and the die of a stretch-forming machine, stretching the sheet metal to approximately 25% of its yield strength, and driving the bulldozer to about 75% of its final depth. At this point it is conventional to remove the preform and subject it to solution heat treatment. The heat treated preform is then reloaded in the stretch-forming machine, the bulldozer is set at its previous depth, the preform is stretched again to about 25% of yield, the bulldozer is taken to 100% of its design depth, and finally the material of the workpiece is again stretched, this time for an additional 1xc2xd to 3% of yield. These procedures are time consuming and expensive and the same have generally been thought to be limited to use in connection with pieces of sheet metal no greater in size than about 48xe2x80x3 by about 60xe2x80x3and to a draw of no more than about 8xe2x80x3. The prior art procedures have also been thought to be limited to shapes which are not overly complex. These limitations, which are well known to the routineers in the stretch draw forming art, are imposed as a result of the difficulty in handling larger preforms and because of the tendency for friction lock to occur when the desired shapes become too complex. The prior art process have also been limited to use with heat treatable materials because it has not been possible using conventional techniques to produce final shapes which have been sufficiently work hardened. That is to say, those skilled in the art have previously believed that parts which are 260xe2x80x3 long could not be formed using stretch draw methodology. Moreover, draws of 20xe2x80x3 or so have been unheard of. These problems have been exacerbated when the materials to be formed are not suitable for being hardened by heat treatment. For example, aluminum to be used in forming a boat must be corrosion resistant, and corrosion resistant aluminum is not amenable to heat treatment hardening.
The problems encountered in prior art processes discussed above are ameliorated if not eliminated completely through the use of the present invention which provides a stretch draw process for shaping a workpiece initially comprising an elongated metal sheet having a pair of spaced opposite ends. The process involves a completely new series of steps. Initially the metal sheet to be formed is placed in a metal forming position in a draw stretch forming machine between a bulldozer having a female forming surface and a die having a male forming surface. Preferably the sheet is positioned such that said ends thereof project laterally outwardly beyond the forming surfaces of the bulldozer and die. The forming surfaces are complemental and configured to draw the metal sheet into a predetermined shape. The bulldozer is moveable relatively toward the die from a draw initiating position where it is adjacent the metal sheet to a fully closed metal sheet shaping position. The draw initiating position and the fully closed position are spaced apart a predetermined distance. Initially the bulldozer is moved from a fully open position where access is provided for loading the work piece into the machine. The bulldozer is then moved into its draw initiating position where it just touches the metal sheet. The ends of the metal sheet are then pulled outwardly to eliminate the slack and position the same in its correct position for being formed. During this slack removing procedure, the metal sheet is preferably not stretched.
The metal sheet work piece is partially drawn toward is desired shape by moving the bulldozer toward the die with the work piece therebetween. During this operation the bulldozer is moved from its draw initiating position to a working position located between the draw initiating position and its fully closed position. In accordance with the invention, the partially drawn work piece is stretched by pulling outwardly on the ends thereof after the bulldozer has reached its working position, and the stretching is continued until any puckering or gathers in the work piece are relieved. After the puckering and/or gathers are relieved, the drawing of the work piece is completed by moving the bulldozer toward the die with the relieved work piece therebetween from the bulldozer working position to bulldozer fully closed position to thereby give the workpiece its final predetermined shape. The shaped work piece is stretched again after the bulldozer has reached its fully closed position by pulling outwardly on the ends thereof to thereby give the shaped work piece memory, set its shape, and work harden the same.
In a preferred form of the invention, the bulldozer working position may be closer to the fully closed position than it is to the draw initiating position. Preferably, the ratio of the distance between the working position and the draw initiating position to the distance between the working position and the fully closed position ranges from about 1.5:1 to about 19:1, more preferably from about 2.33:1 to about 9:1, even more preferably from about 4:1 to about 7:1 and ideally such ratio is approximately 5.67:1.
In a particularly preferred form of the invention, the metal of the metal sheet work piece is a corrosion resistant aluminum, and ideally is 5052 aluminum. Specifically the invention is particularly useful for forming boat hull components. For these purposes, it is preferred that the metal sheet work piece is at least about 15 feet long, and ideally the same may be approximately 20-25 feet long.